Geologic formations defining a reservoir for the accumulation of hydrocarbons in the subsurface of the earth contain a network of interconnected paths and pores through which fluids can flow from the formation into a borehole and from the borehole into the formation. To determine the nature and behavior of the fluids in the network, knowledge of both the nature of the pore fluids and the porosity of the geologic formations is desired. With this information, efficient development and management of hydrocarbon reservoirs may be achieved.
For example, the electrical resistivity of geologic formations is a function of both porosity of the formations and resistivity of the fluids. Considering that hydrocarbons are electrically insulating and most formation water is saline and thereby electrically conductive, resistivity measurements provide valuable data to infer the amount of water present in hydrocarbon reservoirs in geologic formations. Based on resistivity measurements it is further possible to monitor the changes in hydrocarbon content as production of the hydrocarbon proceeds and water content increases.
Details on methods and tools for determining resistivity of the formation in the space between two or more wells can be found for example in the two articles, “Crosshole electromagnetic tomography: A new technology for oil field characterization”, The Leading Edge, March 1995, by Wilt et al. and “Crosshole electromagnetic tomography: System design considerations and field results”, Society of Exploration Geophysics, Vol. 60, No. 3, 1995 by Wilt et al. Both sources describe the measurement of geologic formation resistivity employing low frequency electromagnetic (EM) systems with sources and receivers in two boreholes. Further methods and tools for performing EM measurements are described in a number of patents and patent applications including the co-owned U.S. Pat. No. 6,393,363 issued to Wilt and Nichols.
Similar measurements however performed from within a single well are less well known. Examples of such single-well EM surveys are published in: Alumbaugh, D. L., and Wilt, M. J., “A numerical sensitivity study of three dimensional imaging from a single borehole”: Petrophysics, 42, 19-31 (2001) and M. Wilt and et al., “3D extended logging for geothermal resources: Field trials with the Geo-Bilt system”, PROCEEDINGS, Twenty-Seventh Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, Calif., Jan. 28-30, 2002. The latter publication suggests an extended tool design for single-well EM surveys.
More typical for single-well measurements are the known well logging methods. Named after the physical phenomenon exploited to perform the measurement, these characterizing methods for the well and its immediate surrounding are referred to as acoustic or sonic, resistivity and induction logging. All of these methods include one or more receivers and transmitters mounted on a logging tool and are typically limited to probing the close vicinity of the borehole. One of the factors limiting the reach or characterizing depth of these methods is the maximal available transmitter-receiver spacing.
To increase the reach of sonic or acoustic waves, Schlumberger has developed for example a wireline tool (referred to as the Borehole Acoustic Reflection Survey or BARS tool) that allows reservoir features such as reflectors and fractures to be imaged from a single borehole. The BARS tool builds on existing sonic logging technology with the distance between source and receiver section being made variable by the introduction into the tool of spacer sections, the length of which is determined by prior simulation of the reservoir features. Further information on the BARS tool can be found in co-owned U.S. Pat. No. 6,956,790 issued to J. Haldorsen and the literature found therein.
Though it is known that the depth of investigation of tools can be theoretically increased by increasing the distance between transmitters and receivers, the length of the logging tools cannot be arbitrarily increased to accommodate the larger distance between receivers and transmitters. Typically the maximum transmitter-receiver spacing on a tool in a single well is 10 m or less. In tools specifically designed to measure parameters deep in the reservoir, this distance may reach up to 35 m.
In a specific field of seismic exploration, the so-called Vertical Seismic Profiling (VSP) process, a number of tools have been proposed aiming at isolation sensors suspended into the borehole from noise traveling along it whilst maintaining at the same time a good contact with the wall of the borehole. Examples of such tools can be found in U.S. Pat. No. 4,578,785 issued to V. Gelfand, U.S. Pat. No. 5,259,452 issued to C. Wittrisch, and U.S. Pat. No. 7,048,089 issued to P. West et al.
In view of the known art, it is seen as one object of the invention to improve and enhance single well logging tools and methods. It is seen as a particular object of the invention to increase the depth of investigation for known logging methods without increasing the length of the logging tool. It is seen as another object of the invention to improve existing methods and tools for single-well EM tomography.